The present invention relates generally to the field of nip press technology and, more particularly, to systems and methods for measuring nip width between loaded rolls in a nip press.
In the process of papermaking, many stages may be required to transform headbox stock into paper. The initial stage is the deposition of the headbox stock onto paper machine clothing or felt. Upon deposition, the white water forming a part of the stock flows through the interstices of the felt, leaving a mixture of water and fiber thereon. The felt then supports the mixture, leading it through several dewatering stages such that only a fibrous web or matt is left thereon.
One of the stages of dewatering takes place in the nip press section of the papermaking process. In the nip press section, two or more cooperating rolls press the fibrous web as it travels on the felt between the rolls. The rolls, in exerting a great force on the felt, cause the web traveling thereon to become flattened, thereby achieving a damp fibrous matt. The damp matt is then led through several vacuum and dewatering stages.
The amount of pressure applied to the web during the nip press stage may be important in achieving uniform sheet characteristics. Variations in nip pressure can affect sheet moisture content and sheet properties. Excessive pressure can cause crushing of fibers as well as holes in the resulting paper product. Conventional methods addressing this problem have been inadequate, and thus, this problem persists in the nip press stage, often resulting in paper of poor quality, having uneven surface characteristics.
Roll deflection, commonly due to sag or nip loading, is a source of uneven pressure distribution. Rolls have been developed which monitor and alter the roll crown to compensate for such deflection. Such rolls usually have a floating shell which surrounds a stationary core. Underneath the floating shell are pressure regulators which detect pressure differentials and provide increased pressure to the floating shell when necessary.
Notwithstanding the problem of roll deflection, the problem of uneven loading across the roll length, and in the cross machine direction, persists because pressure is often unevenly applied along the roll. For example, if roll loading in a roll is set to 200 pounds per inch, it may actually be 300 pounds per inch at the edges and 100 pounds per inch at the center.
Conventional methods for determining a pressure distribution profile for a roll may involve the use of nip width sensors. Nip width measurements may be taken along the length of a roll and then processed to generate a pressure distribution profile. Unfortunately, conventional technology uses spaced sensors configured in such a way that they may not be able to detect certain nip width variations that occur over small spatial dimensions, such as those near the end of a nipped roll or those near regions having high temperatures. For example, the region just outside the sheet edge of calendar covers may be especially important. There is no sheet to insulate the calendar cover from the heated mating roll and the dub has not started to keep the roll materials separated. Thus, the calendar cover in the region just outside the sheet edge may become hot and expand radially due to thermal expansion. This region may have greater nip pressure because it is similar to a localized thicker region. Another example is where local damage or a local hot spot raises the cover.